Maintenance-free and corrosion-resistant to dirt and cleaning with dry ice

The investment in this cap changer for robot welding tongs is worth it. The system made extremely reliable by a purely mechanical design is only used in the automotive industry. It helps to reduce production costs in the long term. Robust linear guide systems with polymer sliding elements withstand the harsh usage and surrounding conditions.

For several years, “DryLin® W” linear plain bearings from the bearing specialist igus® GmbH, Cologne, have been used in mechanical cap changers. The design of the guide system allows extremely high flexibility in the construction and a simple installation by the application of individual or double rails. Hard-anodized aluminium is used as rail material and provides very good friction and wear results. As no lubricants are needed, the whole system is resistant to dirt. Double rails in size 16 mm are used in the application. “In the meantime we have already fitted 1,400 of these linear guide systems. They are robust and work so well that so far we haven't had to order a single replacement part,” says Michael Geisler.

“DryLin®” involves an extensive range of maintenance and lubricant-free linear plain bearings in four main different groups. Besides not needing oil and grease, the focus is also on the robustness and resistance to factors like dirt, water, chemicals, heat and impact. These benefits come into their own in the automotive industry among others. The modular “DryLin®” kit is also continually growing, meaning that ever new application fields can be opened up.

The company REU-Schweißtechnik GmbH was founded in 1985 with a patented process for recycling electrode caps. Since 1995 the company has been providing training in the area of resistance welding and also for MSG-welding since 2002. Its third main business is bringing welding tongs into service and optimising their parameters. “And for a few years our range has included the mechanical electrode cap changer. This in-house development is suitable for all robot welding tongs,” the technical manager explains. “The investment is relatively modest compared to motorized versions,” says Geisler. “Our system works without its own drive, we use the robot available. We achieve the best possible success with this solution. “

And this is how it works: The robot receives a message from the welding controls that the electrode's service life has been reached. The welding controls sends a signal via the PLC that a cap change is necessary. The robot moves to the cap changer, loosens the caps, fetches the new caps from a magazine, puts them in place and can continue its production process after 25 to 40 seconds. “Our aim is to make full use of the service life of electrode caps. Production therefore hardly ever stops,” points out Michael Geisler.

Electrode welding caps are pure consumables. They are changed every 2,000 to 10,000 spots depending on the application. Two caps are used in each electrode holder. As a rule, six changes a day take place, meaning that twelve electrodes are replaced per day on each holder. Many automotive manufacturers still change the caps by hand during breaks and downtimes on the transfer lines, whereby manual changes often cause damage and hence increased wear on the electrode shafts. In addition the procedure takes valuable time. The time saved can therefore be used for more worthwhile activities such as maintenance and monitoring tasks. “The change does only take a few minutes, but with cycle times of around 30 seconds in car body construction and six changes a day, you can soon work out how soon our appliance pays for itself,” Michael Geisler explains. The cost factor is in the low four-figure area and comes to approx. a third compared to driven systems.

To compensate for tolerances with the holder movement, the company first used elastic rubber buffers, which were fitted relatively rigidly. “After developing the system further, we now use linear technology, which not only guides in a defined manner, but can also withstand forces and always goes back to the starting position,” says the technical manager. “They must also be resistant to external influences, such as dirt and dust. ” The linear plain bearings must work OK with both magnetic fields, which emerge during welding, and with various cleaning agents, such as dry ice.

Robust linear technology with polymer gliding elements As the welding systems are usually cleaned wet or with dry ice, one of the main issues on linear guide systems is corrosion-resistance. The rails, the housing bearings and the gliding elements must overcome these demands. This is where the incorporated lubrication in the “iglidur®” gliding elements pays off. The solid lubricants are embedded as microscopic particles in the millions of minute chambers of the solid, mostly fiber-reinforced material. The bearings release minute amounts of lubricants from these chambers. That is adequate to sufficiently lubricate the immediate surrounding.

The solid lubricants assist in reducing the coefficient of friction of the iglidur® bearing. They are not indispensable for the bearing's function, but have a supporting effect. As they are embedded in the minute chambers, they cannot be shoved aside. They always make their presence felt as soon as the bearing or shaft is set in motion. The lubricant is protected as long as possible from external contamination and can therefore constantly release its full effect. In this example, “iglidur® J200” is used as a material, which is a specialist for very low coefficients of friction and minimal wear on anodized aluminium shafts. Saving on raw materials, which includes the environmental issue, also plays an important role in this context. Just as the welding caps are changed precisely, the guide system does not need any additional lubrication either, which ultimately has a positive effect on the environment. “You can rely on the system permanently over the long term. No manpower has been necessary since the installation to maintain the guide system,” Michael Geisler points out.